Ophthalmoscope Optical Layout

by | FAQ, Opthalmology, Victor Argueta

In the next couple of articles, we will talk about the different instrumentation used by ophthalmologists and optometrists.  As you can imagine, there are many restrictions when designing an optical instrument that will be used in a patient.  It matters whether the instrument is meant to be used as a diagnostic tool or during surgery, and whether the patient is awake or unconscious.

For this first set of articles, we will focus on optical instruments used in the diagnosis and evaluation of some human eye parameters.  Some of you may actually have been required to use them when doing a regular eye exam.

For today’s entry, we will talk about one of the most basic optical instruments used by optometrists and family doctors:  the ophthalmoscope.  This instrument allows the doctor to see the back of the eye.  They can observe the retina, the optic nerve, the macula fovea, and the vascular system (all these structures are sometimes referred to as the fundus of the eye).  By looking at these structures, a doctor can diagnose different ailments like endocarditis, diabetes, glaucoma, separation of the retina, and hypertension.

There are three models of ophthalmoscopes”: classical, panoptic, and indirect

ophthalmoscope (that looks like a miners’s light).  The first two are shown in Figure 1.

Ophthalmoscope Optical Layout 1

Figure 1.  Classic and Panoptic ophthalmoscope. Image from Standford Medicine

Optical Design

An ophthalmoscope consists of 3 essential elements: a light source, a mirror or beam splitter that redirects the light to the eye, and an optical means of correcting an non sharp image of the fundus.  In reality, they can be more complex.  You normally want to control the amount of light and the size of the light source \ to adapt the light cone that goes into the patient’s eye.   In addition, you may need different colors of light source to enhance contrast or identify retinal lesions. Some ophthalmoscopes include grids as a basic measurement between retinal features.  In order to correct the non sharp fundus image, one can use a wheel of lenses with different powers or a movable lens that allows to focus at different points. An image of a ophthalmoscope is shown in Figure 2.

Ophthalmoscope Optical Layout 2

Figure 2. Design of a direct ophthalmoscope.  Image from Delhi Journal of Ophthalmology

Finally, I would like to mention that when performing a test, an optometrist may require you to dilate your pupils.   This is not always the case but it will depend on how much of the retina they need to observe.

Related Posts